Tidy mass scaling in solid mechanics

[WilkAndy]

Motivation

In explicit solid-mechanics simulations, to ensure that stiff/small/light regions of the mesh don't force the critical time-step to be inordinately small, inertial mass scaling is standard practice. Tidy this existing functionality.

Design

• This appears to be done by the DensityScaling Material. It needs a few tweaks. Eg:
  • Perhaps it is suboptimally named, as it actually provdes a density addition, and does not actually scale the original density. So, perhaps rename it to DensityAddition.
  • Or, change it so that it outputs an actual scaled density that can be used in explicit time stepping
• There appears to be no input file that illustrates how to use mass scaling in practice

Impact

Hopefully better naming means it's easier for users to understand things